Multiple choice

A motor boat covers a certain distance downstream in 30 minutes, while it comes back in 45 minutes. If the speed of the stream is 5 kmph what is the speed of the boat in still water?

  1. 10 kmph

  2. 25 kmph

  3. 20 kmph

  4. 15 kmph
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Let boat speed be B and stream speed be S = 5. Downstream speed = B+5, Upstream speed = B-5. Distance = (B+5) * (30/60) = (B-5) * (45/60). 0.5(B+5) = 0.75(B-5). B+5 = 1.5(B-5). B+5 = 1.5B - 7.5. 0.5B = 12.5. B = 25 kmph.

AI explanation

Let the speed of the boat in still water be x kmph and the total distance be D kilometers. Using the formula Time equals Distance divided by Speed, the downstream equation is D divided by the sum of x and 5 equals 0.5 hours, and the upstream equation is D divided by the difference of x and 5 equals 0.75 hours. Equating the distance D from both gives 0.5 multiplied by the sum of x and 5 equals 0.75 multiplied by the difference of x and 5, which simplifies to 2x plus 10 equals 3x minus 15. Solving for x yields a boat speed of 25 kmph.

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